1. Field of the Disclosure
The disclosure relates to ultra-high power fiber laser systems emitting a MW-level peak and kW-level average power output substantially in a fundamental mode. Particularly, the disclosure relates to a single mode fiber laser system with a booster stage configured with as an active multimode fiber which has a few centimeter-long output core region end-pumped in a counter-propagating direction.
2. Prior Art
The dramatic rise in output power from rare-earth-doped fiber sources over the past decade, via the use of double clad fibers led to a range of fiber-laser system with outstanding performance in terms of output power, beam quality, overall efficiency, and wavelength flexibility. Yet the power scaling of modern high power fiber laser systems is far from satisfying ever increasing industry demands.
Currently, advances in this field are primarily constrained by limitations in maximum extractable energy, and the onset of nonlinear effects. Saturation energy of the gain medium is a key parameter for determining how much energy can be stored in an amplifier, and is given by
                              E          sat                =                                            hv              s                        ⁢                          A              eff                                                          (                                                σ                  es                                +                                  σ                  as                                            )                        ⁢                          Γ              s                                                          (        1        )            whereas, as are the emission and absorption cross section at the signal wavelength, h ½s is signal energy at frequency ½s, Aeff is area of the active doped region and s is signal overlap with the active region.
The deleterious nonlinear effects and particularly stimulated Brillouin scattering (SBS) and stimulated Raman scattering (SRS) rob power from the signal and can cause catastrophic damage. As one of ordinary skill in the fiber laser arts knows, mitigation is possible by increasing the modal area and decreasing the fiber length. Because a larger core occupies a larger fraction of the overall fiber cross-section and therefore has higher pump absorption, the optimum fiber length varies inversely with Aeff. Thus, increasing the core area naturally results in shorter length.
However, the core cannot be limitlessly increased. For single-mode operation, as the core diameter increases, the refractive index difference between the core and cladding, n, must decrease and, after a certain threshold, become bend sensitive. And when n is fixed at a minimum, further increase in core diameter results in multimode operation. While this is permissible, core size is then constrained by unavoidable but undesirable energy transfer among modes. The mode coupling efficiency between modes in a multimode fiber is given byη˜(λ2k2)/(Δn2peff)  (2)where k is the perturbation amplitude due to index and microbead fluctuations, neff is the difference in effective indices between different modes, and p is a fitting parameter (with value>0) to account for mechanical perturbations on a fiber. Thus, large neff is desirable for low mode coupling. Unfortunately, as Aeff increases, neff decreases and at a certain point the mode coupling cannot be avoided.
An additional problem with large Aeff designs in all applications of high power lasers and amplifiers involves spatially transforming and focusing the device output. This is best achieved with Gaussian beams. Thus, an important metric for high power devices is a measure of the departure from a perfect Gaussian spatial profile M2 (M2=1 is a perfectly Gaussian mode).
Current preferred laser designs concentrate on means to provide operation in a fundamental mode with a low M2, even though the fiber may guide several modes. One disclosed means to achieve this is to design an amplifying system including multiple separate fibers which are fused to one another. In particular, the system is configured with a uniformly dimensioned SM passive fiber guiding SM signal light to a taper configured to adiabatically expand the MFD of the SM to a size substantially matching the size of a fundamental mode of uniformly dimensioned MM amplifying fiber which is fused to the output of the taper. Because of multiple fiber components, splice losses may be unforgivably high. Furthermore, manufacturing the multicomponent system, as discussed above, is time inefficient and thus costly. Still further, low threshold of NLEs and bending losses may still be unacceptable.
Recently, the fiber laser industry has turned to crystal fiber rods typically used in output stages of amplifier chains to address the scalability of fiber amplifiers. Based on airclad technology, a crystal fiber rod includes a clad structure surrounding a large diameter core which is capable of supporting substantially only a fundamental mode. The core is configured with a very small numerical aperture (“NA”) which typically does not exceed about 0.02.
In fiber rods, like in any double-clad structure, pump light is coupled into an inner cladding, which has an adequate NA. With a clad pumping configuration and low dopant concentration, typical lengths of fiber rods are about one meter and longer. Such fiber lengths have a few undesirable consequences, as explained below.
An open-end structure of fiber rods may pose certain problems. Typically, launching signal light throughout air gaps can be realized by micro-optics. The latter complicates the entire configuration making it cumbersome and cost-ineffective. The presence of air in the gaps or holes lowers thermal conductivity. In particular, it slows dissipation of heat, which, in turn, may damage the rod itself and be environmentally hazardous.
As one of ordinary skill knows, the premise underlying an efficient high power, single mode fiber laser system, is rather simple: maximally enlarged mode field diameter of the fundamental mode and the shortest possible MM doped core. These desired characteristics of a high power fiber laser system can be easily understood by the presence of nonlinear optical effects (“NLE”), which are considered critical limitations for achieving MW power peak levels and high quality laser beam laser outputs.
In general, NLEs, which in fact include diverse physical phenomena, can be presented as
            N      ⁢                          ⁢      L      ⁢                          ⁢      E        =                  ∫        0        l            ⁢                                    P            ⁡                          (              z              )                                A                ⁢                  ⅆ          z                      ,wherein I is a the fiber length, P is the power (in pulsed systems the peak power), A is the in-core guided mode field area, z is the position of power along the fiber.
However, increasing the core doped region in MM waveguides leads to increasing the number of guided modes higher than the fundamental mode and, as a consequence, to degrading the beam quality. This can be mitigated by the core's greatly reduced numerical aperture (“NA”) as has been implemented in the above mentioned fiber rods. The small NA critically limits the amount of pump light coupleable into the core leaving clad pumping as the only viable option.
As to the known pumping arrangements, whether the pump light is coupled into the fiber rod's cladding in either of forward- or back-propagation direction in accordance with the end-pumping scheme or by using a side pumping scheme, the length of a fiber rod necessary for the desired absorption is greater than 50 centimeters. Such a length, even if the above mentioned core/clad area ratio is high, inevitably leads to a low NLE threshold.
Turning now to power sources, the brightness of MM pump laser diodes, which in light of absence of high power single mode (“SM”) laser diodes are necessary, may not be adequate. If the available brightness of the pump is somewhat acceptable, the need for improved absorption of pump light, which leads to decreased fiber lengths, can be realized by the increased fiber core/cladding area ratio in double clad fibers, as featured in fiber rods. However, the increased ratio may reduce the brightness acceptance in the fiber cladding.
Summarizing the above, the design of high power fiber systems faces difficult challenges because of the following factors: nonlinear effects in fibers in general and fiber rods in particular, loss of fundamental mode power to high order modes (“HOM”); pump brightness and, of course, excessive heat generation. Although each factor limits power scaling independently, in the booster stage (the final gain stage), they are also interrelated, i.e., reducing one may increase the effects of another.
A need therefore exists for an ultra-high power fiber laser system substantially overcoming the above-discussed disadvantages of the known systems.